[lworld] RFR: C++ preparation for preview mode

[David Beaumont]

On Mon, 22 Sep 2025 22:21:25 GMT, David Beaumont <duke at openjdk.org> wrote:

> C++ changes for supporting preview mode *when* preview mode resources (with new location flags) are available.
> 
> At the moment, this code will operate on non-preview jimage files (1.0) and act as if no preview resources are available by virtue of the default value for missing attributes being zero (which matches location flags for "normal" entries).

src/java.base/share/native/libjimage/imageFile.hpp line 253:

> 251:         // Set on a "normal" (non-preview) location if a preview version of
> 252:         // it exists in the same module.
> 253:         FLAGS_HAS_PREVIEW_VERSION = 0x1,

Also set on package directories if any of its entries have preview content.

src/java.base/share/native/libjimage/imageFile.hpp line 256:

> 254:         // Set on all preview locations in "/modules/xxx/META-INF/preview/..."
> 255:         FLAGS_IS_PREVIEW_VERSION = 0x2,
> 256:         // Set on a preview location if no normal (non-preview) version of

Also set on package directories if all its entries are preview only, but the C++ code doesn't need to know that because it's never dealing with those entries.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1609#discussion_r2370805859
PR Review Comment: https://git.openjdk.org/valhalla/pull/1609#discussion_r2370800884